1. Field of the Invention
The present invention relates to a constant velocity universal joint for use in a power transmission system of a vehicle, various industrial machines, and the like, and also to a wheel bearing device for rotatably supporting a driving wheel to a vehicle body using the same.
2. Description of the Related Art
A wheel beating device of a vehicle, as shown in FIG. 6 as an example, includes a wheel hub 1, a double-row bearing 2, and a constant velocity universal joint 3b, as essential components. In the bearing device, the constant velocity universal joint 3b is press-fitted into an inner periphery of the wheel hub 1 in order to allow torque transmission. FIG. 5 shows an outer joint member 12 that constitutes a conventional constant velocity universal joint 3b assembled in the wheel bearing device In FIG. 6. Components for the wheel bearing device having the following constructions.
The wheel hub 1 has an outboard side inner race 4 formed on its outer peripheral surface, and a flange 6 for attachment of a wheel (not shown). Hub bolts 7 for fixing a wheel disc are inserted at equal intervals in a circumferential direction of the flange 6. An inner ring 9 is fitted onto a smaller diameter stepped portion 8 of the outer peripheral surface of the wheel hub 1 on the inboard side end. An inboard side inner race 5 is formed on the outer peripheral surface of the inner ring 9. A serration 10 is formed on the inner peripheral surface of the through hole of the wheel hub 1.
To prevent a creep, the inner ring 9 is press-fitted with an adequate amount of interference. The outboard side inner race 4 formed on the outer peripheral surface of the wheel hub 1 and the inboard side inner race 5 formed on the outer peripheral surface of the inner ring 9 are used as double-row inner races. The inner ring 9 is press-fitted onto the smaller diameter stepped portion 8 of the wheel hub 1, and an outer joint member 12 of the constant velocity universal joint 3, inserted from the inboard side of the wheel hub 1 in the axial direction, is then fastened onto the wheel hub 1 to mate the end portion of the inner ring 9 with the shoulder portion 13a of the outer joint member 12 so as to prevent the inner ring 9 from coming off and to perform a pre-load control.
An outer ring 14 has outer races 15 and 16 on its inner peripheral surface, to face the inner races 4 and 5 of the wheel hub 1 and the inner ring 9, respectively, and a flange 17 for attachment onto a vehicle body (not shown). This flange 17 is fixed to the knuckle (not shown) extended from a suspension apparatus of the vehicle with bolts.
The bearing 2 is a double-row angular contact ball bearing, and has rollers 18 interposed between the inner races 4 and 5, and the outer races 15 and 16. The inner races 4 and 5 are formed on the outer peripheral surfaces of the wheel hub 1 and the inner ring 9, respectively. The outer races 15 and 16 are formed on the inner peripheral surface of the outer ring 14. The rollers 18 in the respective rows are held with a cage (not shown) at equal intervals in a circumferential direction. A pair of seals 19 and 20 are fitted to the inner periphery of the outer ring 14 at both open ends of the bearing 2 to seal an annular space between the outer ring 14, and the wheel hub 1 and the inner ring 9 to prevent grease filling inside the bearing from leaking and to prevent intrusion of water and foreign matters from outside.
The constant velocity universal joint 3 includes an outer joint member 12 having a track groove 21 formed on an inner peripheral surface, an inner joint member 23 having a track groove 22 formed on an outer peripheral surface opposing to the track groove 21 on the outer joint member 12, balls 24 interposed between the track groove 21 on the outer joint member 12 and the track groove 22 on the inner joint member 23, and a cage 25 retaining balls 24 between the outer joint member 12 and the inner joint member 23.
The outer joint member 12 includes a bowl-shaped mouth portion 26a accommodating the inner joint member 23, the balls 24 and the cage 25, and a stem portion 27 integrally formed with and extending from the mouth portion 26a in an axial direction with a serration 11 formed on its outer peripheral surface. To fix the constant velocity universal joint 3 to the wheel hub 1, the stem portion 27 is inserted into the through hole of the wheel hub 1, so that the outer peripheral surface of the stem portion 27 and the inner peripheral surface of the through hole are mated with the serrations 11 and 10 which are formed thereon, respectively, and a nut 29 is set to the thread portion 28 formed in the shaft end portion and tightened. A pre-load is controlled by applying an axial force produced by tightening the nut 29.
As shown in FIG. 5, the outer joint member 12, which is a part of the above-mentioned constant velocity universal joint 3, includes the mouth portion 26a and the stem portion 27. The mouth portion 26a has the plurality of track grooves 21 formed on its inner peripheral surface, and a shoulder portion 13a formed on the outer peripheral surface corresponding to its bottom portion. The stem portion 27 has a back face 30, which is an end face of the shoulder portion of the mouth portion 26a, to be abutted to an end portion of the inner ring 9, a thread portion 28 and the serration 11 that enables torque transmission between the wheel hub 1 and the stem portion 27. The stem portion 27 is fixed to the wheel hub 1 by tightening the nut 29 on the thread portion 28, thereby mating the serration 11 with the wheel hub 1 to transmit the torque.
Accordingly, a base portion 31 of the stem potion 27 is subjected to a tensile stress caused by tightening the nut 29 to the tread portion 28, a shearing stress caused by the torque transmission through the serration 11, and a bending stress by the wheel hub 1 to which the wheel is mounted. Since the base portion 31 of the stem portion 27 must have a strength enough to endure this compound stress, the base portion 31 is designed to have enough wall thickness in the back face 30 and is subjected to surface hardening treatment.
However, an increase in the wall thickness in the back face 30 can lead to increase in the weight of the constant velocity universal joint 3b, which is not preferable. Sine it also locates the center of the outer joint member 12 as near as possible to the center of king pin, restriction is imposed on the design of the base portion 31 of the stem portion 27 so as to select only the shape to be continued from the serration 11 to the back face 30 through a chamfer. This design limitation will cause stress concentration both at the serration 11 and at the base portion due to the chamfer shape, to decrease the strength. One possible countermeasure against this problem is to make an outside diameter of the base portion 31 of the stem portion 27 larger. However, this countermeasure cannot be regarded as good means because it requires a drastic change in design of the vehicle wheel portions.
In general, a region ranging from the serration 11 of the stem portion 27 to the shoulder portion 13a of the mouth portion 26a through the back face 30 of the base portion 31 (induction hardened area A) and a track region which is an inner peripheral surface where the track groove 21 of the mouth portion 26a is formed (induction hardened area B) are subjected to surface hardening treatment by induction hardening. The wall of the back face 30 is designed to be thinner because the base portion 31 of the stem portion 27 is subjected to induction hardening. However, since the base portion 31 is chamfered, the depth of the hardened portion becomes shallow in the base portion 31. This depth becomes shallower more significantly when the outside diameter of the shoulder portion 13a is equal to or more than twice of the outside diameter of the serration in the stem portion 27.
This is because the base portion 31 is difficult to be hardened for fear of a quench crack and a fusion at the corner portion 32. The quench crack and the fusion may occur when the serration 11 of the stem portion 27, the back face 30 of the base portion 31 and the shoulder portion 13a of the mouth portion 26a are subjected to induction hardening at the same time because it is difficult to concentrate the heat at the base portion 31 of the stem portion 27, while it is easy to concentrate the heat at the corner portion 32 between the back face 30 and the corner portion 13a. The quench crack is easier to occur in induction hardening than in other heat treatments, because heating up time to a high temperature (900 to 1000.degree. C.) is so short as a few seconds and so is a cooling time.
A possible countermeasure against this problem is to use a ferrite core and the like for the base portion 31 of the stem portion 27 to facilitate a heat concentration at the base portion 31 during induction hardening. However, in practice, it is difficult to attain a good heat concentration at the base portion 31. Another possible countermeasure is to chamfer the shoulder portion 13a of the mouth portion 26a to an obtuse angle. However, this may significantly deteriorate sealing performance.
Another possible countermeasure to improve the strength of the base portion 31 of the stem portion 26a is to change the material of the outer joint member 12 to alloy steel. However, this cannot be an effective countermeasure because it substantially deteriorates forgeability. Another possible countermeasure is to improve fatigue strength by shot peening. However, this hardly improves a static strength and impact strength.
Prevention of quench crack at the corner portion 32 between the back face 30 and the shoulder portion 13a requires so many quality control items including check of clearance between an induction heating coil and the outer joint member 12, heat control during heating the coil, frequency optimization, coolant concentration control, cooling start time optimization, and Jominy value control for a material, that they actually cause a low yield rate and a high cost.